Phase-shifting cell for an antenna reflector

ABSTRACT

The invention relates to phase-shifting cells constituting the passive reflectarrays of antennas with a reconfigurable transmission direction, transmitting in the microwave range. More particularly, the invention describes, within the context of phase-shifting cells of the type having dipole strands angularly distributed in a star configuration, a novel type of switch consisting of a microelectromechanical device essentially comprising a suspended micromembrane which, under the action of an electrostatic force caused by a control voltage, deforms sufficiently to ensure electrical connection between the strands, making it possible to form a dipole in the desired orientation. In one particular embodiment, the micromembrane can be likened to one of the plates of a capacitor and its deformation corresponds to a substantial increase in the capacitance of this capacitor, thus providing the electrical connection. This switch technology has the advantages of greater fabrication simplicity and of enhanced performance compared with the known technologies. The invention provides the main geometrical and technological characteristics for obtaining optimized performance.

The field of the invention is that of passive reflectarrays composed ofa mosaic of elementary phase-shifting cells for an antenna with areconfigurable transmission direction, operating in the microwave range.

In a large number of applications, it is necessary to be able to pointthe electromagnetic beam transmitted by an antenna in the desireddirection. The possible applications are in particular:

-   -   space telecommunications: tracking of an area on the ground in        the case of an orbiting satellite, minimization of the        interfering radiation when there is simultaneous use of several        signals, reprogramming of the antenna owing to a change in        traffic and in-flight redundancy in order to alleviate defective        antennas;    -   applications on board aircraft: aircraft-satellite        communications and radar applications;    -   ground applications: millimeter wave communications and        meteorological radar applications.

To obtain this orientation, there are three possible techniques. Firstlyit is possible to mechanically orient the entire antenna in the desireddirection. This solution requires mechanical positioning devices thatare complex to operate in the case, for example, of space applications.In a second solution, an antenna called an active antenna is produced,this being composed of a plurality of elementary transmitting cells. Bycontrolling the phase of the various signals transmitted by each cell,transmission in the desired direction is obtained. However, thissolution, although more flexible than the previous one, has thedrawbacks of being expensive and heavy.

The third technical possibility is illustrated in FIGS. 1 and 2 andconsists in producing an antenna from a single transmitting source 1supported by an arm 2, which illuminates a reflectarray 3. The wholeassembly is controlled by an electronic signal control module 5. Thereflectarray is composed of a mosaic of passive phase-shifting cells 4generally arranged in a honeycomb, which retransmit a beam in thedesired direction. To control the direction of retransmission, it istherefore sufficient to control the phase shift introduced by each cell.This solution has, like the active antenna, the advantage of notrequiring moving parts. However, it does not have any of its drawbacks,the operation of a single powerful source being simpler and lessexpensive to implement than the operation of a multitude of independentsources.

There are several solutions for producing elementary phase-shiftingcells. A first solution consists in making the wave of wavelength λpropagate along, and causing it to be reflected in, a waveguide of givenlength L. The phase shift Φ introduced is then proportional to the ratioL/λ. The desired phase shift is thus obtained by adapting the length ofthe waveguide. This phase shift also depends, by the same principle,directly on the wavelength of the transmitted signal and consequentlythis type of device can only operate over narrow spectral transmissionbands.

To alleviate this drawback, one type of device allows a phase shiftwhose value is practically wavelength-independent to be obtained (JamesP. Mongomery: A Microstrip Reflectarray Antenna Element—AntennaApplications Symposium—Sep. 20-22, 1978, pp 1-16, University ofIllinois). This device is suitable for waves transmitted in circularpolarization.

The basic principle of this type of device is shown schematically inFIGS. 3 and 4. The phase-shifting cell principally comprises a planedielectric substrate 6 of thickness equal to about one quarter of thecentral operating wavelength, on which are deposited, on the lower part,a ground plane 10 and, on the upper part an even number of conductingdipole strands 7 arranged in a regular fashion around a central disk 8,which is also conducting. Switching devices 9 are used to connect, onthe command, two diametrically opposed strands to the central disk. Whentwo strands are thus connected to the disk, they constitute a radiatingdipole having a given geometrical orientation, the other, non-connected,strands not radiating or only very slightly.

The operating principle is the following: let there be a circularlypolarized wave incident upon a phase-shifting cell, two of the strandsof which are connected to form a dipole. It may be demonstrated that ifthe electric field vector representing this circular wave makes, at thesurface of the dipole, a phase-shift angle +θ with the direction of saiddipole, then the transmitted electric field will make, with thedirection of the dipole, a phase-shift angle −θ. Depending on thedipoles created in each phase-shifting cell, it thus becomes possible tocontrol the phase shift introduced and consequently the angle ofretransmission of the beam. The major advantage of this arrangement isthat the phase shift introduced is thus virtually independent of thewavelength of the signal.

One of the main technological difficulties with this type ofphase-shifting cell is the production of the switching devices. Eachreflectarray may comprise several tens of phase-shifting cells andconsequently several hundred switching devices. They therefore have tobe reliable, to be small in size—typically the size of each switch mustnot exceed a few hundred microns—, to have a low power consumption theymust not interfere with the operation of the microwave dipole.

Patent U.S. Pat. No. 5,835,062 (Flat panel-configured electronicallysteerable phased array antenna having spatially distributed array offanned dipole sub-array controlled by triode-configured field emissiondevices) proposes to produce the switches from electronic triodes. Thissolution requires the production and implantation, for each triode, of aswitch consisting of conical microcathodes and annular microanodes. Tooperate, these devices also require substantial electrical power owingto the large number of switches per reflectarray.

As to the invention, this proposes an alternative solution that makes itpossible to simplify the production of the device and to reduce theelectrical power consumed. The object of the invention to produce theswitches from microelectricalmechanical devices.

The principle of operation of this type of device is illustratedschematically in the FIGS. 5 and 6 in the simplest case of the use as amicroswitch. A metal membrane or beam 11 of very small thickness is heldsuspended by supports 14 above conducting surfaces 12 and 13 that aremutually isolated. The membrane/conducting surfaces assembly may besubjected to an electrical voltage T. In the absence of applied voltage,the membrane is suspended above the conducting surfaces and there is noelectrical contact between them. In this case, an electrical currentcannot flow between 12 and 13 and the membrane/conducting surfacesassembly is likened to an open switch. When the membrane/conductingsurfaces assembly is subjected to an increasing voltage T, the membraneis subjected to an electrostatic force that deforms it until themembrane comes into contact with the conducting surfaces for a voltageT_(C). The electrical current can then flow from 12 to 13. Themembrane/conducting surfaces assembly is then equivalent to a closedswitch. Thus, a microswitch is produced. The main advantages of thistype of device are essentially:

-   -   the production techniques, which are derived from conventional        technologies for the thin-film fabrication of microelectronic        circuits, technologies which make it possible to achieve low        production costs compared with other technologies, while still        guaranteeing high reliability;    -   the very low consumed electrical power levels, which are        virtually zero;    -   the overall size. Thus, a microswitch can be produced in an area        of the order of one tenth of a square millimeter; and    -   the performance in microwave operation. This type of switch has        very low insertion losses, of the order of one tenth of a        decibel, much less than those of devices providing the same        functions.

More precisely, the subject of the invention is a phase-shifting cell ofa reconfigurable reflectarray for an antenna operating in the microwaverange, said array comprising a plurality of phase-shifting cells, eachof said phase-shifting cells being composed of several electricallyconducting strands, characterized in that at least two of said strandsmay be connected together by means of at least one switching devicecomprising a microelectromechanical system comprising an electricallycontrollable flexible membrane, the strands thus connected constitutinga radiating dipole.

Within the context of reflectarrays whose geometrical arrangement of thestrands is in the form of a star, said phase-shifting cell comprises twoplane parallel faces separated by a thickness representing about onequarter of the wavelength of the operating frequency, said first facehaving a star-configured array consisting of an even number ofelectrically conducting strands that are all identical and placeduniformly around a central disk, which is also conducting, it beingpossible for each strand to be electrically connected to the centraldisk via a switching device dependent on a control voltage, each pair ofdiametrically opposed strands thus constituting, when the two devicesconnecting them to the central disk are activated, a resonant dipole inthe range of operating frequencies of the antenna, the second faceconsisting of a ground plane, said cell being characterized in that theswitching device consists of a microelectromechanical system comprisinga flexible membrane supported by at least two pillars that are placedbetween said membrane and the first face of the cell, said membrane thusbeing placed above the end of each strand facing the central disk andthat peripheral part of said disk which is placed facing this end, saidmembrane, when the control voltage is applied, being deformed by theresulting electrostatic force sufficiently to ensure electricalconnection between the end of the strand and the correspondingperipheral part of the central disk.

Advantageously, the switching device is of the capacitor type and theelectrical connection corresponds to a large increase in itscapacitance. Operation of the microswitch as a simple switch withelectrical contact between the flexible membrane and the elements of thedipole has the drawback of having a very low reliability. In theoperating frequency range considered, the use of a microcapacitor of lowcapacitance, typically varying from 1 femtofarad in open circuit to 1picofarad in the closed circuit makes it possible to obtain excellentclosed-position coupling and a very good open-position isolation, whileconsiderably increasing the reliability of the device.

Advantageously, the ratio of the value of the capacitance of thecapacitor in the absence of a control voltage to the value of thecapacitance when the control voltage is applied is of the order of 100.In this case, the plates of the capacitor consist, on the one hand, ofthe flexible membrane and, on the other hand, of the end of the strandand of the peripheral part of the corresponding disk that are placedbeneath this membrane, electrical isolation being provided by a layer ofdielectric material covering the strands and the disk. This material ispreferably silica nitride. The geometrical and mechanical parameters ofthe membrane are designed in such a way that the control voltage to beapplied, in order to ensure switching, is large compared with thepossible parasitic voltages. This control voltage is typically thirtyvolts. The reliability of the device, the switching time and the controlvoltage depend partly on the geometrical characteristics of themembrane. The best compromise is obtained when the membrane takes theshape of a rectangular parallelepiped of small thickness, the width ofthe rectangle typically being one hundred microns, its length threehundred microns and its thickness seven hundred nanometers. Thematerials used for producing the membrane are advantageously gold,aluminum or tungsten titanium alloys deposited in layers. In the absenceof a control voltage, the plates of the capacitor are separated by aboutthree microns.

Advantageously, the end of the strand and the facing part of the centraldisk that are placed beneath the membrane make up a comb ofinterdigitated fingers and the total number of fingers is preferablyfive. The shape, in the form of interdigitated combs, of the twosurfaces of the end of the strand and of the facing central disk allowthe capacitive effect to be optimized.

The voltages for controlling the switching devices pass via the strandsby means of internal resistive lines and the flexible membranes are allconnected to the electrical ground, also by means of other internalresistive lines. The material used to produce the various electricalconnections is preferably gold. The value of the impedance of theresistive lines at the operating frequency is high enough to isolate allthe strands, the central disk and the switching devices from theoutside.

Advantageously, the cell is of hexagonal shape and comprises twelvestrands, each strand preferably having a flared shape, the flare anglebeing about 20 degrees. The hexagonal shape of the cell allows completeand uniform paving of the reflectarray space. On principle, the phaseshift introduced by each cell is discrete, the minimum phase shift anglebeing inversely proportional to the number of strands. Of course, it isadvantageous to reduce this angle by increasing the number of strands.However, this is limited by the complexity of the interconnectionsystems when the number of strands to be controlled increases, by thenecessary miniaturization limit of the switches, and by the possibleinter-strand interference if they are tightly spaced. In practice,having twelve strands per cell is a good compromise betweentechnological complexity and minimum phase shift angle. The dipole wavereflection coefficient depends on the size of the dipole, which isconventionally close to one half-wavelength, but also on its shape,slightly flared shapes being well suited for obtaining good resonance ofthe dipole.

Advantageously, the electronic system of said cell, formed by thestrands, the central disk, the switching devices and the variousresistive lines supplying the control voltages and the electricalground, is implanted on a microwave-transparent substrate; the materialused may be silicon or quartz or glass, especially glass with the Pyrexbrand name. Said substrate takes the form of a right cylinder with planeparallel faces, of circular or hexagonal base centered on the centraldisk of the cell.

Advantageously, the upper parts of the substrates, which comprise thecentral disks and the various switching devices, are protected by one ormore protective covers. Each cell may have its own protective cover, orthe cover may be a single one, common to the entire reflectarray. Theswitching devices, which are mechanical parts of very small dimensionsof the order of a few microns to a few hundred microns, require a coverfor protecting them from external elements such as fluids or dust, whichwould incur the risk of greatly degrading their performance. Inparticular, the performance of the metal membranes may be seriouslyimpaired by oxidation.

Advantageously, the substrate common to the reflectarray system has twoparallel plane faces, the upper face bearing the various glasssubstrates corresponding to each cell, and the opposite face having aground plane, the material of this substrate being amicrowave-transparent and electrically insulating material. Preferably,this material is based on PTFE and glass fibers. Neltec sells a materialof this type under the brand name METCLAD.

Advantageously, each cell is connected by a honeycomb paving of circularconnection holes that are produced in the common substrate and arrangedin hexagons, each hexagon being centered on a central disk of the cell,each of the internal resistive lines of a cell that emanate from thestrands or from the membranes being connected to these holes via otherexternal resistive connections implanted on the common substrate, theinternal resistive lines implanted on the glass substrates of each cellbeing connected to the external resistive lines implanted on thesubstrate of the reflectarray by means of wire-bonding connection wires.

Advantageously, the rows of connection holes are common to two adjacentcells and each hexagon of connection studs then has a number of studsequal to at least twice the total number of strands of each cellincreased by two, so as to be able to connect two adjacent cells.

It is necessary to ensure isolation of each cell so that a given cellconfiguration does not interfere with the surrounding cells. Thisisolation is provided in two ways: firstly, by the connection holes,which act as an electromagnetic barrier if their spacing is small enoughcompared with the wavelength, and secondly by sets of metal separatingwalls arranged in a hexagon above the connection holes, said walls beingconnected together and grounded via metal centering pins located, on oneside, in the walls and, on the other side, in certain connection holesreserved for this purpose. The set of walls of the cells then forms ahoneycomb grid lying above the reflectarray.

Advantageously, the entire reflectarray is covered with a multilayerdielectric treatment for increasing the effectiveness of the cell whenthe angle of incidence of the incident or reflected radiation is high.

In general, the process for producing the reflectarray comprises thefollowing steps:

-   -   production of the printed circuit substrate, common to the cells        by:        -   deposition of the ground plane and        -   production of the electrical connection studs,            plated-through holes and metallized pads;    -   production of the central microelectronic substrates of the        cells;    -   deposition of the various electronic devices on these substrates        by:        -   production of the strands, the central disk and the            resistive lines; and        -   production of the switching devices;    -   protection of the switching devices by installing covers;    -   installation of the central substrates on the common substrate;    -   electrical connection of the resistive lines to the connection        studs;    -   installation of centering pins;    -   placing of the isolating grids on the centering pins.

Advantageously, the process for producing the switches comprises thefollowing substeps:

-   -   deposition of a layer of dielectric material at the location of        the interdigitated combs;    -   deposition of a layer of photoresist covering at least the        location of the membrane and of its support pillars;    -   removal of said resist at the location of each pillar;    -   creation of the pillars and of the membrane by deposition of one        metal layer at the locations of said pillars and of the        membrane; and    -   removal of the resist at least beneath the membrane so that the        membrane on these pillars is left free.

The invention will be more clearly understood and other advantages willbecome apparent on reading the description that follows, given by way ofnon-limiting example and with reference to the appended figures inwhich:

FIG. 1 shows the basic principle of an antenna according to theinvention;

FIG. 2 shows a top view of the reflectarray, illustrating the hexagonalpaving of the phase-shifting cells;

FIG. 3 shows the general principle of each phase-shifting cell withstar-configured dipoles seen from above. In this view, the switches areshown by simple switches. In normal operating configuration, only twodiametrically opposed switches are closed, the others being left open;

FIG. 4 shows the same diagram as the previous figure, but in crosssection.

FIG. 5 shows the operating principle of a switch based on anelectromechanical device when it is in the OFF position, that is to saywhen there is no potential difference between the membrane and theconducting surfaces lying beneath it;

FIG. 6 shows the operating principle of a switch based on anelectromechanical device when it is in the ON position, that is to saywhen a sufficient potential difference exists between the membrane andthe conducting surfaces lying beneath it, so that mechanical contact isestablished;

FIG. 7 shows a top view of two switching assemblies according to theinvention. Shown in this figure are just the end of two strands facingthe central disk, that part of the central disk facing them, theresistive connections and the membrane of each switch;

FIG. 8 shows a view of the end of the strand and of the facing part ofthe central disk, illustrating the interdigitated combs lying beneaththe membrane. For the sake of clarity, only the outline of the membranehas been shown by dotted lines;

FIG. 9 shows a perspective view of the two switches of FIG. 7, one ofthe two switches being in the OFF position (straight membrane) and theother being in the ON position (curved membrane);

FIG. 10 shows a top view of the cell according to the invention. For thesake of clarity, the switches are shown by dotted lines in the OFFposition and by a solid line in the ON position;

FIG. 11 shows a first sectional view of the cell according to theinvention through the center of the cell. For the sake of clarity, theswitches have not been shown in this figure;

FIG. 12 shows a second sectional view of the cell according to theinvention through the periphery of the cell, illustrating the connectionof a metal wall to the common substrate; and

FIG. 13 shows the general arrangement of the three adjacent cells inplan view.

FIG. 7 shows a top view of the switching devices according to theinvention. Two adjacent conducting strands 7 of a phase-shifting cell 4are shown, together with that part of the central disk 8 facing them.The switching region of each strand is formed by the end of the strandlocated opposite the central disk. The switching device essentiallycomprises a membrane 11 placed above the switching region. The controlvoltages and groundings are applied by means of resistive lines 151, 154and 155.

FIG. 8 shows a detailed view of the switching region. That end 71 ofeach strand placed on the side facing the central disk and thatcorresponding part 81 of the disk placed facing this end make up a combof interdigitated fingers. The region of this comb constitutes theswitching region. The advantage of this geometrical arrangement is thatit allows the control voltage coming from the strand to be distributeduniformly in the switching region. As an example, FIG. 8 shows fiveinterdigitated fingers, two belonging to the central disk and threebelonging to each strand. The entire switching region is covered with alayer of insulating material such as, for example, silica nitride, notshown in the figure.

FIG. 9 shows a perspective view of the two switches shown in FIG. 7.Each membrane is supported by at least two pillars 14 placed on eitherside of the switching region. The membrane is thus isolated at a certaindistance above the switching region. This distance is typically a fewmicrons. Said metal membrane has roughly the shape of a parallelepiped.This shape represents a good compromise between mechanical strength ofthe membrane, which determines its lifetime and its reliability, and thevoltages needed to be applied in order to obtain switching, whichvoltages must not be too high. Thus, for a membrane having a typicallength of three hundred microns, a typical width of one hundred micronsand a thickness of seven hundred nanometers, the control voltages arearound thirty volts. The membrane is also perforated by a multitude ofholes 110 during its production. These holes allow flow of the solventfor freeing the membrane during the production process. For the sake ofclarity, these holes are not shown in the various figures illustratingthe membrane, except in the detailed view in FIG. 7. The membrane ismade of metal. The metals and alloys possible are preferably gold,aluminum, tungsten or titanium.

The assembly consisting of the membrane and the end of the strand andthat part of the central disk lying beneath them form the plates of acapacitor, the rest capacitance of which is a few femtofarads. When themembrane is stressed it deforms, approaching the two plates of thecapacitor. Its capacitance increases and its value then becomes a fewpicofarads.

FIGS. 10, 11 and 12 show the top view and two sectional views of a cellof the reflectarray according to the invention.

FIG. 10 shows the top view of the cell. The central part of the cell 4comprises a substrate 61 on which the star-configured array of theelectrically conducting strands 7 constituting the various dipoles isimplanted, said array being centered on an electrically-conductingcentral disk 8. The substrate is electrically insulating and transparentto the microwaves. It must be compatible with the technologies forimplanting the various electronic components of the cell. For example,this substrate is made of silicon or quartz or glass, especially glasswith the Pyrex brand name. There is necessarily an even number ofstrands, these being arranged symmetrically so that each strand has adiametrically opposed partner. Each pair of diametrically opposedstrands thus constitutes a dipole when it is connected to the centraldisk via the switching devices shown in FIGS. 7, 8 and 9.

The control voltages and groundings are applied by means of resistivelines 151, 154 and 155 connected, on one side, to the various strandsand to the switching membranes and, on the other side, to connectionpads 161 placed around the perimeter of the central substrate. A firstseries of control lines 151 is connected to the end of each strand, asshown in FIG. 10. Two diametrically opposed grounding lines 154 connecttwo membranes to ground, the other membranes and the central disk areconnected to these two membranes via other resistive lines 155, as shownin FIG. 10. The resistive lines 151, 154 and 155 have a high enoughresistance for all of the strands and the switching devices to be fullyelectrically isolated from the microwaves. Typically, the resistivelines deposited have an ohmic resistance of a few hundred ohms/square.

The strands preferably have a flared shape so as to increase theefficiency of the dipole. The flare angle is about twenty degrees. Thelength of each strand is about one quarter of the operating microwavewavelength. The central substrates corresponding to a given cell areimplanted in a regular manner on a substrate 62 common to all the cells4 of the reflectarray. This substrate is also electrically insulatingand microwave-transparent. It must be compatible with the technologiesfor implanting the various electronic components of the cell. Thissubstrate is produced especially from a composite based on PTFE andglass fibers. This type of material is sold by Neltec under the brandname METCLAD. The total thickness of the common substrate and of eachcentral substrate is about one quarter of the operating microwavewavelength, i.e. around one to two millimeters given the operatingfrequencies. This substrate has, on the opposite side from that of thecentral substrates, a ground plane 10.

The common substrate includes a paving of electrical connection studs171 and 172 arranged in a regular fashion in a hexagonal pattern. Eachhexagon is centered on a central cell substrate, as indicated in FIGS. 7and 13, and is composed of six rows of at least six connection studs.The studs of each row are uniformly spaced apart. They pass rightthrough the common substrate (see FIG. 12).

Each cell is surmounted by a set of six metal walls 18 (see FIG. 12)which are also arranged in a hexagon and placed above the rows ofconnection studs, the whole assembly forming a honeycomb grid (see FIGS.10 and 13).

There are two types of stud. The first type is used to connect theresistive control lines to the outside of the reflectarray, toward theelectronic control module, and are isolated from the ground plane. Thesecond type is used, on the one hand, to mechanically fasten the metalwalls to the common substrate, by means of the fastening pins 172, and,on the other hand, to connect these walls to the ground plane, asindicated in FIG. 12.

The studs of the first type are connected to the resistive lines 151 and154 of the common substrates via other resistive lines 153interconnected by means of wire-bonding connection wires 152, asindicated in FIG. 10. Said resistive lines 153 have a high enoughresistance for all of the strands and the switching devices to be fullyelectrically isolated from the microwaves. Typically, the resistivelines deposited have an ohmic resistance of about 1 kilohms/square. Thestuds are isolated from the metal walls by insulating pads 173. Thearrangement of the resistive lines connected to the interconnectionstuds is indicated in FIGS. 10 and 13. This arrangement makes itpossible not only to have the same geometrical arrangement for all thecells of the reflectarray but also to minimize the lengths of theresistive lines.

The switching devices have to be protected as they are mechanicallyfragile. This protection is provided, i.e. at each cell, by a protectivecover 19 as indicated in FIG. 11, which shows a sectional view of thecell. This cover 19 must also be microwave-transparent. It may also becommon to the entire reflectarray.

The central substrates may also be covered with a multilayer dielectrictreatment so as to increase the efficiency of the cells at a high angleof incidence.

The principle of operation of the reflectarray is the following:

-   -   to obtain reflection of the microwaves delivered by the        transmitter in a specified direction, the electronic module        calculates, for each cell, the geometrical arrangement of the        dipoles to be activated;    -   for each cell, the electronic module generates the control        voltages that are sent to the two diametrically opposed strands        to be activated; and    -   under the effect of the voltage, the two membranes placed above        the activated strands deform (see FIG. 9). The capacitance        existing between the plates greatly increases. The order of        magnitude of the ratios of the capacitances in the two states of        the switch is about one hundred. The impedance of the switching        device becomes negligible and the two pressed strands are        connected to the central disk, thus forming a dipole.

The switching devices are operated simultaneously for two opposedstrands by two separate voltage control signals, the geometry of thedevice not allowing the two strands to be connected to the central disksimultaneously by a common control signal.

In general, the process for producing the reflectarray comprises thefollowing steps:

-   -   production of the printed circuit substrate, common to the cells        by:        -   deposition of the ground plane and        -   production of the electrical connection studs,            plated-through holes and metallized pads;    -   production of the central microelectronic substrates of the        cells;    -   deposition of the various electronic devices on these substrates        by:        -   production of the strands, the central disk and the            resistive lines; and        -   production of the switching devices;    -   protection of the switching devices by installing covers;    -   installation of the central substrates on the common substrate;    -   electrical connection of the resistive lines to the connection        studs;    -   installation of centering pins;    -   placing of the isolating grids on the centering pins.

The process for producing the switches comprises the following substeps:

-   -   deposition of a layer of dielectric material at the location of        the interdigitated combs;    -   deposition of a layer of photoresist covering at least the        location of the membrane and of its support pillars;    -   removal of said resist at the location of each pillar;    -   creation of the pillars and of the membrane by deposition of one        metal layer at the locations of said pillars and of the        membrane; and    -   removal of the resist at least beneath the membrane so that the        membrane on these pillars is left free.

1. A phase-shifting cell of a reconfigurable reflectarray for an antennaoperating in the microwave range, said reflectarray comprising: aplurality of identical elementary phase-shifting cells, each of saidcells having two plane parallel faces separated by a thicknessrepresenting about one quarter of the wavelength of the operatingfrequency; said first face having a star-configured array including aneven number of electrically conducting strands that are all identicaland placed uniformly around a central disk, which is also conducting; itbeing possible for each strand to be electrically connected to thecentral disk via a switching device dependent on a control voltage; eachpair of diametrically opposed strands thus constituting, when the twoswitching devices connecting them to the central disk are activated, aresonant dipole in the range of operating frequencies of the antenna,the second face including a ground plane, said cell each switchingdevice including a micro-electromechanical system comprising a flexiblemembrane supported by at least two pillars that are placed between saidmembrane and the first face of the cell, said membrane thus being placedabove the end of each strand facing the central disk and that peripheralpart of said disk which is placed facing this end, said membrane, whenthe control voltage is applied, being deformed by the resultingelectrostatic force sufficiently to ensure electrical connection betweenthe end of the strand and the corresponding peripheral part of thecentral disk, said switching device being of the capacitor type and theelectrical connection corresponds to a large increase in itscapacitance.
 2. The phase-shifting cell as claimed in claim 1, whereinthe ratio of the value of the capacitance of the capacitor in theabsence of a control voltage to the value of the capacitance when thecontrol voltage is applied is of the order of
 100. 3. The phase-shiftingcell as claimed in claim 1, wherein the plates of the capacitor includeone of the flexible membrane and, on the other hand, of the end of thestrand and of the peripheral part of the corresponding disk that areplaced beneath this membrane, electrical isolation being provided by alayer of dielectric material covering the strands and the disk.
 4. Thephase-shifting cell as claimed in claim 3, wherein the dielectricmaterial used is preferably silica nitride (Si₃N₄).
 5. Thephase-shifting cell as claimed in claim 1, wherein the geometrical andmechanical parameters of the membrane are designed in such a way thatthe control voltage to be applied, in order to ensure switching, islarge compared with the possible parasitic voltages.
 6. Thephase-shifting cell as claimed in claim 5, wherein this control voltageis typically thirty volts.
 7. The phase-shifting cell as claimed inclaim 5, wherein the membrane has roughly the shape of a rectangularparallelepiped of small thickness, the width of the rectangle typicallybeing one hundred microns, its length three hundred microns and itsthickness seven hundred nanometers.
 8. The phase-shifting cell asclaimed in claim 5, wherein the membrane and the pillars that support itconsist mainly of gold or aluminum layers or layers of tungsten titaniumalloys.
 9. The phase-shifting cell as claimed in claim 2, wherein, inthe absence of a control voltage, the space between the membrane andthose parts of the central disk and of the strand that are placedbeneath it is about three microns.
 10. The phase-shifting cell asclaimed in claim 1, wherein the end of the strand and the facing part ofthe central disk that are placed beneath the membrane make up a comb ofinterdigitated fingers.
 11. The phase-shifting cell as claimed in claim10, wherein the total number of fingers is five.
 12. The phase-shiftingcell as claimed in claim 1, wherein the voltages for controlling theswitching devices pass via the strands by means of internal resistivelines and in that the flexible membranes are all connected to theelectrical ground, also by means of other internal resistive lines. 13.The phase-shifting cell as claimed in claim 12, wherein the materialused to produce the various electrical connections is preferably gold.14. The phase-shifting cell as claimed in claim 12, wherein the value ofthe impedance of the resistive lines at the operating frequency is highenough to isolate all the strands, the central disk and the switchingdevices from the outside.
 15. The phase-shifter device as claimed inclaim 1, wherein the cell is of hexagonal shape and comprises twelvestrands.
 16. The phase-shifting cell as claimed in claim 1, wherein eachstrand has a flared shape, the flare angle being about 20 degrees. 17.The cell as claimed in claim 1, wherein the electronic system of saidcell, formed by the strands, the central disk, the switching devices andthe various resistive lines supplying the control voltages and theelectrical ground, is implanted on a central microwave-transparentsubstrate, this substrate being especially made of silicon or quartz orglass, especially glass with the Pyrex brand name.
 18. The cell asclaimed in claim 1, wherein said substrate takes the form of a rightcylinder with plane parallel faces, of circular or hexagonal basecentered on the central disk of the cell.
 19. The cell as claimed inclaim 1, wherein the upper part of the substrate, which comprises thecentral disk and the various switching devices, is protected by aprotective cover transparent to the operating microwave electromagneticwaves.
 20. The cell as claimed in claim 1, wherein the substrate commonto the reflectarray system has two plane parallel faces, the upper facebearing the various central substrates corresponding to each cell, andthe opposite face having a ground plane.
 21. The cell as claimed inclaim 20, wherein the substrate is based in particular on PTFE and glassfibers, this substrate possibly being the material having the brand nameMETCLAD, sold by Neltec.
 22. The cell as claimed in claim 1, whereineach cell is connected by a paving of circular connection studs that areproduced in the common substrate and arranged in rows forming a hexagon,each hexagon being centered on the central disk of each cell, each ofthe internal resistive lines of a cell that emanate from the strands orfrom the membranes being connected to these studs via other externalresistive connections implanted on the common substrate, the internalresistive lines implanted on the central substrates of each cell beingconnected to the external resistive lines implanted on the substrate ofthe reflectarray by means of wire-bonding connection wires.
 23. The cellas claimed in claim 22, wherein each hexagon of connection studs has anumber of studs equal to at least twice the total number of strands ofeach cell, increased by two.
 24. The cell as claimed in claim 22 whereinthe rows of connection holes are common to two adjacent cells.
 25. Thecell as claimed in claim 1 wherein each cell is surmounted by a set ofsix metal separating walls arranged in a hexagon above the connectionholes, said walls being connected together and grounded via metal pinslocated, on one side, in the walls and, on the other side, in certainconnection holes reserved for this purpose such that the set of walls ofthe cells forms a honeycomb grid lying above the reflectarray.
 26. Thecell as claimed in claim 1 wherein the electrical isolation of each cellwith respect to the adjacent cells is achieved, on the one hand, by thepaving with connection holes and, on the other hand, by the metal wallsplaced above each cell.
 27. The cell as claimed in one of claim 1,wherein the entire reflectarray is covered with a multilayer dielectrictreatment.
 28. A process for producing the cell as claimed in claim 1,wherein the step for producing the switches comprises the followingsubsteps: deposition of a layer of dielectric material at the locationof the switching region; deposition of a layer of photoresist coveringat least the location of the membrane and of its support pillars;removal of said resist at the location of each pillar; creation of thepillars and of the membrane by deposition of at least one metal layer atthe locations of said pillars and of the membrane; and removal of theresist at least beneath the membrane so that the membrane on thesepillars is left free.
 29. The process for producing the cell as claimedin claim 17 wherein it comprising the following steps: production of theprinted circuit substrate, common to the cells by: deposition of theground plane and production of the electrical connection studs,plated-through holes and metallized pads; production of the centralmicroelectronic substrates of the cells; deposition of the variouselectronic devices on these substrates by: production of the strands,the central disk and the resistive lines and production of the switchingdevices; protection of the switching devices by installing covers;installation of the central substrates on the common substrate;production of the resistive lines and their electrical connection to theconnection studs; and placing of the isolating grids on the rows ofconnection studs and installation of the mechanical supports.